Quick Summary
Thermal Physics explains heat energy, temperature, how heat travels, how matter expands when heated, and what happens during changes of state. The key formulae are Q = mcΔT and Q = mL. Board exams always include definitions, differences, examples of heat transfer, and 1–2 numericals.
Key Definitions
- Heat (Q): Energy that flows from a hotter body to a colder body due to a temperature difference. SI unit: joule (J).
- Temperature: A measure of the degree of hotness or coldness of a body. SI unit: kelvin (K); also measured in °C.
- Thermal expansion: Increase in the size (length, area, or volume) of a body when heated.
- Specific heat capacity (c): Heat required to raise the temperature of 1 kg of a substance by 1°C. SI unit: J kg⁻¹ K⁻¹.
- Latent heat (L): Heat absorbed or released during a change of state, with no temperature change. SI unit: J kg⁻¹.
- Latent heat of fusion: Heat needed to convert 1 kg of solid to liquid at its melting point.
- Latent heat of vaporisation: Heat needed to convert 1 kg of liquid to vapour at its boiling point.
Formulae Reference
| Concept | Formula | SI Unit |
|---|---|---|
| Heat energy (temperature change) | Q = mcΔT | joule (J) |
| Change in temperature | ΔT = T_final − T_initial | kelvin (K) or °C |
| Latent heat | Q = mL | joule (J) |
| Calorimetry principle | Heat lost = Heat gained | — |
| Temperature conversion | K = °C + 273 | — |
Heat vs Temperature — Key Distinction
This is the most commonly tested conceptual difference in this chapter.
| Property | Heat | Temperature |
|---|---|---|
| What it is | Form of energy | Measure of hotness/coldness |
| Depends on | Mass, specific heat, ΔT | Average kinetic energy of particles |
| SI Unit | joule (J) | kelvin (K) |
| Measured by | Calorimeter | Thermometer |
| Can it flow? | Yes — from hot to cold | No — temperature is a property, not a flow |
Illustration: A large bucket of water at 40°C has much more heat energy than a small cup of water at 90°C, even though the cup is hotter. The bucket has more mass, so Q = mcΔT is larger for it.
Thermal Expansion — Three Types
When a solid is heated, its particles vibrate more and push each other apart — the solid expands.
Linear Expansion (change in length)
A rod expands along its length. The increase in length is proportional to original length and temperature rise.
Examples and why they matter:
no gaps, the rails would buckle and bend, derailing trains.
cracking.
thermal expansion in summer would cause them to break.
- Railway tracks: Gaps are left between iron rails. In summer, rails expand and if there were
- Bridges: Expansion joints (roller supports) allow steel bridges to expand in summer without
- Electric power lines: Wires are left with some slack (they sag in summer). If pulled too tight,
Area and Volume Expansion
Solids also expand in area and volume, but linear expansion is most tested at Class 10 level.
Liquids: Water is unusual — it contracts on heating from 0°C to 4°C, then expands above 4°C. This is called the anomalous expansion of water and is why ice floats and aquatic life survives in frozen lakes.
Gases: Gases expand the most on heating. A balloon inflated at room temperature shrinks in a cold room — the gas contracts.
Heat Transfer — Three Modes
1. Conduction
Heat transfers through a solid by direct contact, particle to particle, without the actual movement of particles.
Mechanism: Particles at the hot end vibrate more and transfer energy to adjacent particles.
Good conductors: Metals (copper, iron, aluminium) — free electrons help carry heat. Poor conductors (insulators): Wood, plastic, glass, air, wool.
Examples:
- A metal spoon gets hot when left in hot soup — conduction through metal.
- A wooden handle stays cool on a metal pan — wood is a poor conductor.
- Wearing woollen clothes in winter — wool traps air (insulator) and reduces heat loss from body.
2. Convection
Heat transfers through fluids (liquids and gases) by the bulk movement of the fluid itself.
Mechanism: Hot fluid becomes less dense and rises. Cool fluid sinks to take its place. This creates a convection current.
Examples:
air from the sea moves in to replace it — this is a sea breeze.
- Sea breeze: During the day, land heats up faster than sea. Air above land rises; cooler
- Monsoon winds: Large-scale convection over the Indian Ocean and Indian subcontinent.
- Boiling water: Hot water rises from the bottom, cool water sinks — convection currents form.
- Room heater: Heats air near it; warm air rises; cool air from the room moves in.
3. Radiation
Heat transfers as electromagnetic waves (infrared radiation) and requires no medium. It can travel through vacuum.
Examples:
- Heat from the Sun reaches Earth through the vacuum of space — only possible by radiation.
- Sitting near a bonfire — you feel the warmth even through the air without touching it.
- A thermos flask has a vacuum between its walls to prevent heat transfer by radiation.
Black vs white surfaces:
- Black (dark) surfaces are better absorbers and emitters of radiation.
- White (shiny) surfaces reflect radiation and absorb less.
- This is why solar water heaters use black paint and we wear light colours in summer.
Calorimetry — Principle of Heat Exchange
Principle: When two objects at different temperatures are mixed in an insulated container, heat lost by the hotter body equals heat gained by the colder body.
This assumes no heat is lost to the surroundings.
Change of State and Latent Heat
When a substance changes state (solid ↔ liquid ↔ gas), heat is absorbed or released without any change in temperature.
Heating Curve — What Happens When Ice Is Heated
- Segment going up: temperature rises (Q = mcΔT applies)
- Flat segment at 0°C: ice melts to water — temperature stays at 0°C (Q = mL_fusion)
- Segment going up again: water temperature rises from 0°C to 100°C
- Flat segment at 100°C: water boils to steam — temperature stays at 100°C (Q = mL_vaporisation)
Worked Numerical Examples
Example 1 — Basic Q = mcΔT
Problem: How much heat is needed to raise the temperature of 2 kg of water from 25°C to 75°C? (Specific heat capacity of water = 4200 J kg⁻¹ K⁻¹)
Example 2 — Calorimetry (Heat Lost = Heat Gained)
Problem: 200 g of water at 80°C is mixed with 300 g of water at 20°C in an insulated container. Find the final temperature of the mixture. (c_water = 4200 J kg⁻¹ K⁻¹ for both)
Example 3 — Latent Heat of Fusion
Problem: Calculate the heat required to melt 500 g of ice at 0°C. (Latent heat of fusion of ice = 3.36 × 10⁵ J kg⁻¹)
Example 4 — Combined: Melting Ice Then Heating Water
Problem: How much heat is needed to convert 200 g of ice at 0°C into water at 60°C? (L_fusion = 3.36 × 10⁵ J kg⁻¹, c_water = 4200 J kg⁻¹ K⁻¹)
Example 5 — Finding Final Temperature After Mixing (Different Materials)
Problem: A 0.5 kg iron block at 100°C is placed in 1 kg of water at 25°C in an insulated container. Find the final temperature. (c_iron = 450 J kg⁻¹ K⁻¹, c_water = 4200 J kg⁻¹ K⁻¹)
Board Exam Q&A — Model Answers
One Mark Questions
Q: Define heat. A: Heat is a form of energy that flows from a hotter body to a colder body due to a temperature difference between them. SI unit: joule (J).
Q: What is temperature? A: Temperature is the measure of the degree of hotness or coldness of a body. It is a measure of the average kinetic energy of the particles of the body. SI unit: kelvin (K).
Q: Write the formula for heat energy. A: Q = mcΔT, where Q = heat (J), m = mass (kg), c = specific heat capacity (J kg⁻¹ K⁻¹), and ΔT = change in temperature (K or °C).
Q: What is latent heat? A: Latent heat is the heat absorbed or released by a substance during a change of state without any change in temperature.
Two Mark Questions
Q: Differentiate between heat and temperature.
| Property | Heat | Temperature |
|---|---|---|
| Nature | Form of energy (flows) | Property (measures hotness) |
| SI Unit | joule (J) | kelvin (K) |
| Depends on | Mass, specific heat, ΔT | Average kinetic energy of particles |
| Measured by | Calorimeter | Thermometer |
Q: Why are gaps left in railway tracks? A: Iron rails expand when the temperature rises in summer. If no gaps were left, the expanding rails would push against each other, buckle, and bend — making them dangerous for trains. The gaps (expansion joints) accommodate the thermal expansion and keep the tracks straight.
Q: Name and explain the three modes of heat transfer. A: 1. Conduction: Heat transfer through solids by particle-to-particle vibration, without bulk movement of matter. Example: A metal spoon heating up in hot soup. 2. Convection: Heat transfer through fluids by bulk movement of the fluid itself. Hot fluid rises, cool fluid sinks. Example: Sea breeze during the day. 3. Radiation: Heat transfer as electromagnetic waves through vacuum or air. No medium needed. Example: Heat from the Sun reaching Earth.
Five Mark Questions
Q: Explain the change of state of water from ice to steam with a temperature-heat graph.
When ice at 0°C is heated continuously:
1. Stage 1 (ice heating): Temperature of ice rises from below 0°C to 0°C as heat is added. Q = mcΔT applies with c_ice = 2100 J kg⁻¹ K⁻¹.
2. Stage 2 (melting — flat region at 0°C): All heat goes into breaking bonds in ice crystal. Temperature stays at 0°C until all ice melts. Q = mL_fusion (L = 3.36 × 10⁵ J kg⁻¹).
3. Stage 3 (water heating): Water temperature rises from 0°C to 100°C. Q = mcΔT with c_water = 4200 J kg⁻¹ K⁻¹.
4. Stage 4 (boiling — flat region at 100°C): All heat goes into converting liquid to vapour. Temperature stays at 100°C until all water becomes steam. Q = mL_vaporisation (L = 2.26 × 10⁶ J kg⁻¹).
5. Stage 5 (steam heating): Steam temperature rises above 100°C.
The flat portions in the graph (at 0°C and 100°C) are where latent heat is absorbed with no temperature change. This explains why steam at 100°C causes worse burns than water at 100°C — steam releases extra latent heat when it condenses on skin.
Q: Explain thermal expansion with three practical examples. Why does water show anomalous expansion?
Most solids expand on heating because heat increases particle vibration, pushing particles further apart. Liquids expand more than solids; gases expand the most.
Practical examples: 1. Railway gaps: Expansion joints prevent buckling of rails in summer heat. 2. Bridge roller joints: Steel bridges rest on rollers so they can expand and contract freely. 3. Bimetallic strip (thermostat): Two metals bonded together expand at different rates when heated — the strip bends, which opens/closes a circuit. Used in electric irons and fire alarms.
Anomalous expansion of water: Water behaves unusually between 0°C and 4°C — it contracts (becomes denser) as it is heated from 0°C to 4°C. Above 4°C it expands normally. This means water is densest at 4°C.
Why this matters for life: In winter, the surface of a lake freezes at 0°C, but water at 4°C sinks to the bottom (densest). The bottom of the lake stays at 4°C even when the surface is frozen. This allows fish and aquatic life to survive in the warmer water at the bottom.
Common Board Exam Mistakes
is a property (kelvin). They are measured differently.
that is fine, it means heat is released.
must be added for the total heat.
- Treating heat and temperature as the same: Always clarify that heat is energy (joules), temperature
- Calculating ΔT incorrectly: ΔT = T_final − T_initial. If the temperature drops, ΔT is negative —
- Forgetting to convert grams to kilograms: In Q = mcΔT, mass must be in kilograms.
- Skipping units in answers: Always write J for heat energy and °C or K for temperature.
- In change-of-state questions: Forgetting there are two steps (Q = mL then Q = mcΔT) — both
Tamil Nadu Board Exam Pattern
with a numerical, OR three modes of heat transfer with examples.
or explanation (why gaps in railway tracks).
examples above.
- Chapter weightage: 5–7 marks
- Five-mark: Usually a change-of-state explanation with heating curve, OR calorimetry principle
- Two-mark: Definitions (specific heat capacity, latent heat), difference (heat vs temperature),
- Numericals: 1–2 numericals using Q = mcΔT or Q = mL or calorimetry — practice all five
- Tip: The anomalous expansion of water and its ecological importance is a favourite 2-mark question.
Revision Checklist
- Heat vs temperature difference table memorised with units
- Three modes of heat transfer: name + mechanism + one example each
- Q = mcΔT: formula, all symbols and units
- Q = mL: formula, used for change of state
- Heating curve sketch: two flat portions (0°C and 100°C) with reasons
- Calorimetry principle: heat lost = heat gained
- All five worked numericals solved independently
- Anomalous expansion of water explained with ecological importance
- Three practical examples of thermal expansion prepared